One type of lithographic system used in semiconductor manufacture and other applications is known as a "step and repeat" projection aligner. These projection aligners are designed to project a mask pattern onto a layer of resist formed on the wafer. Typically the mask pattern is formed on a mask comprising an opaque layer on a transparent substrate. Following exposure, the layer of resist can be developed to form a resist mask for etching or otherwise patterning the wafer. Typically, the mask pattern can correspond to a die-sized area on the wafer. To pattern the entire wafer, the mask pattern can be stepped-and-repeated across the wafer. This type of projection aligner is also known as a "wafer stepper" because the wafer is stepped, while the mask containing the mask pattern remains stationary.
In addition to "step and repeat" systems, an emerging technology is called "step and scan". In these systems both the mask and wafer "scan" a field, and then the wafer "steps" to the next field. The mask can be scanned across an exposure slit or a slice of a lens. The wafer can be scanned at a speed corresponding to a lens reduction ratio. For example, in a 4.times. reduction system the mask can be scanned at 100 mm/sec and the wafer at 25 mm/sec.
In addition to optical lithographic systems, non-optical systems utilize ion beams (e.g., hydrogen and helium ions), electron beams, and x-rays for exposing a layer of resist on a target. Non-optical lithographic technologies typically employ membrane masks. This type of mask can include a silicon substrate with a thin (2-3 .mu.m) membrane in the center of the substrate. In addition, the membrane can include physical holes that form the mask pattern.
FIG. 1 illustrates a conventional step and repeat optical projection exposure system 10 for exposing a target 12 coated with a layer of resist 11. The projection exposure system 10 includes an exposure source 14 and a mirror 16 adapted to produce collimated exposure energy of a desired wavelength. Typical mercury lamp exposure sources have an operating range of 365 nm to 436 nm (i line). Other exposure sources include KrF excimer lasers at 248 nm, extreme uV (EuV) at 11-13 nm (soft x-ray), and proximity x-ray at 11 angstroms (hard x-ray).
In addition to the exposure source 14, the projection exposure system 10 includes a filter 18 and a condenser lens 20 located along an optical axis 30 of the system. A mask 22 contains a mask pattern 26 to be projected onto the target 12. The mask 22 typically includes a transparent substrate, such as quartz, having an etched opaque layer, such as chrome, that forms the mask pattern 26. In the illustrative exposure system 10 the mask 22 is square. Other systems can employ masks having a generally circular peripheral configuration.
Directing exposure energy through the mask 22 provides energy for exposing the resist 11 on the target 12 to form individual patterned areas 28A-F. Each patterned area 28A-F corresponds to a single exposure of the mask pattern 26. A reduction lens 24 can be located between the mask 22 and the target 12 so that the patterned areas 28A-F on the target 12 are reduced in size with respect to the mask pattern 26. For example, the mask pattern 26 can be five times larger (5.times.) than the resultant patterned areas 28A-F. For forming the individual patterned areas 28A-F, the target 12 can be stepped in x and y directions as required by a wafer chuck (not shown) of the projection exposure system 10. The projection exposure system 10 can also include an alignment system (not shown) configured to accurately align the target 12 with respect to the mask 22 for each exposure step.
FIG. 2A illustrates the mask pattern 26. The mask pattern 26 corresponds in size to a single patterned area 28A-F on the target 12. As shown in FIG. 2A, the mask pattern 26 can include multiple pattern segments "A". Each pattern segment "A" can correspond to an area on the target 12 such as a semiconductor die, or a smaller or larger area, as required.
FIG. 2B illustrates a prior art stepping sequence for the target 12 using the mask pattern 26 (FIG. 2A). Initially, a first exposure step (1st) can be performed through the mask pattern 26. This forms the first patterned area 28A on the target 12 comprising multiple pattern segments "A". The pattern segments "A" on the target 12 correspond to the pattern segments "A" on the mask pattern 26. The target 12 (or the mask 22) can then be stepped through a step distance "SD". The step distance "SD" can be along the "x" axis as shown, or alternately along the "y" axis, or along both the "x" and "y" axes.
Next, a second exposure step (2nd) can be performed through the mask pattern 26. This forms the second patterned area 28B on the target 12 comprising multiple pattern segments "A". The second patterned area 28B is offset from the 1st patterned area 28A by the step distance "SD". The target 12 (or the mask 22) can then be stepped by the step distance "SD", and a third exposure step (3rd) through the mask pattern 26 can be performed. This forms the third patterned area 28C on the target 12 comprising multiple pattern segments "A". The third patterned area 28C is offset from the second patterned area 28B by the step distance "SD", and from the first patterned area 28A by twice the step distance "SD". This step and repeat process can be performed until the entire target 12 has been patterned. The stepping distance "SD" remains the same for each exposure step.
Another prior art step and repeat exposure process is illustrated in FIGS. 3A and 3B. In FIG. 3A a mask pattern 26A-B includes complementary pattern segments "A" and "B". The complementary pattern segments "A" and "B" are formed such that the desired patterned areas on the target 12 achieved by double exposing areas of the target 12 by overlaying the "A" and "B" pattern segments.
As shown in FIG. 3B, during a first exposure step (1st) a first patterned area 28A-B1 can be partially formed on the target 12 (FIG. 1). Next, the target 12 can be stepped through a stepping distance "SD", and a second exposure step (2nd) can performed to partially form a second patterned area 28A-B2. Next, the target 12 can be stepped through the stepping distance "SD" and a third exposure step (3rd) can be performed. The 3rd exposure step completes the second patterned area 28A-B2 and partially exposes the third patterned area 28A-B3. This step and repeat process can be repeated across the target 12 with the stepping distance "SD" being equal for each step. As is apparent the patterned areas at the ends of the target 12 remain incomplete.
The step and repeat process is utilized because in general, it is not practical to produce masks and lenses large enough to pattern a target such as an entire semiconductor wafer with a single exposure step. Accordingly, the size of the target is no longer a limiting factor. This permits semiconductor manufacturers to utilize wafers with increasingly larger diameters (e.g., 200 mm, 300 mm).
One limitation of step and repeat lithographic methods is that misalignment of the mask patterns during the stepping sequences can cause inaccuracies in the resultant patterns on the target. For example, masks are subjected to thermal expansion, and various mechanical stresses during lithographic processes. These factors can cause registration errors during mask fabrication as well as during use of the mask to pattern a semiconductor wafer. Mask writing errors, due to thermal effects and mechanical stage placement errors are one source of misregistration. During use of the mask to pattern a semiconductor wafer, these same thermal and mechanical stresses can cause pattern displacement which is exacerbated over larger distances. It is therefore preferential to have complementary patterns as close to one another as feasible. In addition, alignment and registration errors are compounded when the stepping distances are relatively large due to stage mechanical and interferometry tolerances.
In view of the foregoing, improved projection exposure methods and systems are needed in the art.